Rotary drilling of deep wells for the production of fluid minerals, such as oil and gas, relies upon long assemblies of multiple pipe sections called “strings”. Each pipe unit or section of pipe for this purpose, normally, is in the order of nine to twelve meters (30 to 40 feet) in length and threaded at each end for the making up of the strings.
Drill pipe, which forms the primary pipe string for advancing the borehole depth and often provides rotational torque to the drill bit, is usually fabricated with tapered external threads at one end and tapered internal threads at the opposite end. The external threads of a segment or section of drill pipe are formed into a heavy tool joint called a “pin,” which can be welded to one end of the drill pipe section. The internal threads of the section of the drill pipe are formed into a complementary tool joint called a “box,” which can be welded to the opposite end of the drill pipe section. Notably, drill pipe tool joints have a significantly greater outside diameter than the remaining length of the pipe section.
Drill collars are a special case of thick-walled drill pipe in which the entire length of the thick-walled pipe section has approximately the same outer diameter (O.D.) as the tool joint of the corresponding pipe to provide maximum weight to a pipe section. In theory, drill collars are the operational bearing weight applied to the drill bit. The number of drill collars in a drill string can determine the value of the bearing force applied to the drill bit. The drill string above the drill collars is, theoretically, suspended in tension. Hence, the drill collars are located at the bottom end of a drill string, just above the drill bit.
Oil field casing and tubing pipe are usually formed with external threads at both ends of a pipe section. Two sections of pipe are joined together by a short length “close coupling” having internal threads at opposite ends.
In the course of downhole operations, such pipe strings occasionally become tightly stuck in a well. Typically, the borehole walls of loose or unstable geological strata, which is penetrated by the pipe string, “sluff” or collapse into the borehole around the pipe string and above the drill bit, thus forming a formation seizure. Such a wall collapse may occur for hundreds or even thousands of feet along the borehole length. In such an event, it is impossible to withdraw the pipe string from the borehole or, in most cases, even rotate the pipe string.
Often, it is desirable to retrieve as much of the pipe string above the seizure point as possible. In any case, it is essential to extract the drill string above the seizure point to enable further operations. However, simply reversing the rotation of the pipe string will not necessarily separate the string at the first screwed joint above the seizure. As additional pipe sections are added to a pipe string, the earlier assembled joints become tighter and more difficult to unthread or decouple. Consequently, without some focused intervention, an upper threaded joint will normally disassemble before a lower threaded joint.
Accordingly, drill collars, being at the bottom end of the string, frequently are the most tightly joined. Moreover, it is along the drill collar portion of the drill string that a formation seizure is most likely to occur. Furthermore, due to the massive quantity of alloyed steel present in a drill collar, the drill collars are among the most valuable components of a drill string, thereby adding to the incentive to recover as many of the drill collars as possible.
There are numerous methods and devices for locating the seizure point along a pipe string. A representative existing method and apparatus, disclosed in U.S. Pat. No. 7,383,876, are usable for identifying a specific joint above a seizure point and are incorporated herein by reference. After locating the specific joint above the seizure point, the traditional method, which is used to effect release of the threaded assembly at the specific joint, is to apply a gentle or moderate “left hand” torque to the top or surface end of the pipe string, as the specific joint is jarred by a nearby explosion. However, as discussed below, these traditional methods encounter serious functional and reliability issues when used at great depths, particularly where the environmental conditions include high temperatures and/or high pressures.
Explosive devices for urging the release of screwed joints have heretofore been made in various forms. Typically, a “back-off tool”, as such devices are characterized in the well drilling arts, comprises detonation cord, such as “Primacord”, which is a flexible tube filled with a suitable high explosive, such as HMX, RDX or HNS, that is set off by an electrically initiated detonator. When used under low temperature and pressure conditions prevailing in shallow wells, prior art “back-off” tools and methods have produced generally satisfactory results. However, in extremely deep wells of 6,000 meters (20,000 ft.) or greater, where the temperatures may be in the order of 200° C. (400° F.) or greater and the pressures may be several thousand pounds per square inch, the prior art methods and apparatus encounter serious functional and reliability issues. For example, high pressures tend to decrease the explosive detonation sensitivity and suppress the shock intensity of an explosion, thus causing a complete lack of function or a diminished function of the explosives taught by the prior art methods and apparatus. In addition, high temperatures tend to reduce the energy of the explosive, thus causing a lack of function or a diminished function with regard to the prior art methods and apparatus.
Determining the exact quantity of explosive to detonate against a particular pipe joint, at a particular depth, and in a particular well, remains, to a large degree, a skilled art form. Prior experimentation and testing has led to the development of tabulations of recommended explosive distribution rates for a range of frequently encountered circumstances. Such tabulations of empirically developed data are generally stated in terms of explosive weight values per unit length (e.g., grains of explosive distributed over a foot of length). For example, joints of drill collar sizes that are greater than 19.05 cm (7½ inch), positioned at depths below 2,300 m (7,500 ft.), may require an explosive distribution rate in excess of 300 grams per meter (1400 grains per foot) of length for the uncoupling of the drill collars. However, detonation cord is largely limited to distribution rates of 21 grams per meter (100 grains per foot) of explosive length. Therefore, while the distributed weight value of detonation cord may be increased by detonating multiple parallel cords simultaneously, this technique is largely limited to about a maximum of fourteen (14) detonation cords. Therefore, using 21 grams per meter (100 grains per foot) cords, the use of fourteen (14) cords will allow only 300 grams of explosive per meter (1400 grains per foot) of length. Hence, for releasing 19.05 cm (7½ inch) drill collar joints at greater than 2,300 meters (7,500 ft.) of depth, methods other than multiple detonation cords are required and are necessary to position a sufficient distribution of explosive weight adjacent to the targeted drill collar joint. As such, existing methods and apparatus cannot perform successfully at such great depths.
A need exists for methods and apparatus that are usable for decoupling or unthreading an intended drill collar or pipe joint (i.e., threaded tool joint) from a downhole string of pipe in high temperature conditions, high pressure conditions, and/or at great depths.
A further need exists for determining and providing the exact quantity of explosive needed for detonation against an intended drill collar or pipe joint, at a particular depth and in a particular well, to decouple or unthread the intended drill collar or pipe joint.
The embodiments of the present invention meets all of these needs.